Typically, gas turbine engines include a plurality of injectors for injecting fuel into a combustor to mix with air upstream of a flame zone. The fuel injectors of conventional turbine engines may be arranged in one of at least three different schemes. Fuel injectors may be positioned in a lean premix flame system in which fuel is injected in the air stream far enough upstream of the location at which the fuel/air mixture is ignited that the air and fuel are completely mixed upon burning in the flame zone. Fuel injectors may also be configured in a diffusion flame system such that fuel and air are mixed and burned simultaneously. In yet another configuration, often referred to as a partially premixed system, fuel injectors may inject fuel upstream of the flame zone a sufficient distance that some of the air is mixed with the fuel. Partially premixed systems are combinations of a lean premix flame system and a diffusion flame system.
During operation, fuel is injected into the combustion chamber through the injectors into three or four stages, such as a pilot nozzle, an A-stage, a B-stage, and a C-stage (for configurations having tophat injection or pilot premix features). The pilot nozzle provides fuel that is burned to provide a mini-diffusion flame injector and also provides stability for the premixed A-, B-, and C-stages. Often turbine engines are run using high levels of airflow, thereby resulting in lean fuel mixtures with a flame temperature low enough to prevent the formation of a significant amount of NOx. However, because lean flames have a low flame temperature, lean flames are prone to high CO production. And because excess CO production is harmful, a need exists to limit CO emissions.
Turbine engines often operate at higher fuel to air ratios at partial loads rather than at full load. However, turbine engines are designed for full loads. Thus, nozzles designed to run at full load run excessively lean at partial loads. Inlet guide vanes (IGVs) can be used to reduce air flow through the engine at partial loads, thereby increasing the fuel to air ratio and enabling the engine to operate more efficiently through a larger range of loads. However, IGVs may only be used to restrict air flow a limited amount.
Fuel staging is used to control fuel injection at loads below which IGVs may be used effectively. Fuel staging is a process of emitting fuel from less than all of the injectors in a fuel system. By reducing the number of injectors through which fuel is ejected, the amount of fuel passed through the injectors during operation of the turbine engine at partial loads is increased, and thus, burnout is improved. However, fuel staging creates interfaces between fueled air flows and unfueled air flows. The unfueled air flows quench the flame in the combustor and cause increased production of CO at these fuel/unfueled interfaces. Thus, a need exists for reducing the amount of CO produced by turbine engines using fuel staging at partial loads.